1. Field of the Invention
The present invention relates to a shielding casing for preventing radiation noise radiated from a printed circuit board or a cable in an electronic product, and to an electronic product.
2. Description of the Related Art
In recent years, with the increase in operating speed and the improvement in performance of electronic products, radiation noise radiated from an electronic product affects another electronic product and this becomes a problem. The influence of the radiation noise on the another electronic device is called electromagnetic interference (EMI). The radiation noise causes reception interference in wireless products and communication products and malfunctions of electronic products in typical cases. Therefore individual countries enact regulations on radiation noise from electronic products in the frequency band from 30 MHz to 1 GHz or the frequency band from 30 MHz to 2 GHz, and it is necessary for electronic product makers to design and produce products so as to meet the regulations.
There is generally used a method of covering a noise source such as a printed circuit board, a cable or a module in an electronic product with a shielding casing made of a metal, an electroconductive resin or a resin with a plating in order to reduce the above-described radiation noises and malfunctions due to external electromagnetic waves.
However, it is necessary to provide an opening in the shielding casing as shown in FIG. 10 in order to connect the printed circuit board, cable or module placed in the shielding casing to the outside of the shielding casing. That is, in an office machine such as a copying machine or a printer, an opening is required as an optical path through which an optical signal converted from image information is sent to an optical sensor such as a CCD or a CMOS sensor. An opening formed as an optical path through which an optical signal converted by a semiconductor laser is sent to a photosensitive drum is also required. In some case, an opening formed as a paper path through which a sheet of paper is passed is required. As a condition for passage of light and paper, formation of an opening having a size equal to or larger than a certain size is required.
In FIG. 10, reference numeral 1201 denotes a shielding casing, and reference numeral 1202 denotes a rectangular opening formed for passage of light or paper and having two shorter sides and two longer sides. The length of the longer sides of the opening 1202 is D1 and the length of the shorter sides is D2. Radiation noise (electromagnetic waves) radiated from a noise source such as a printed circuit board placed in the shielding casing leaks out of the shielding casing through the opening 1202. Radiation noise (electromagnetic waves) leaking out through the opening 1202 occurs at all frequencies. The radiation level at each frequency is comparatively low and is not a serious problem.
In the case where the opening 1202 is provided in the shielding casing 1201, however, a signal at a specific frequency corresponding to the shape of the opening 1202 resonates and the opening 1202 itself functions as a slot antenna. That is, radiation noise at the specific frequency is emitted out of the shielding casing 1201 through the opening 1202. Radiation noise emitted through the opening 1202 functioning as a slot antenna occurs only at the specific frequency. However, this radiation noise has a considerably high radiation level and becomes a serious problem in meeting radiation noise regulations.
The principle of functioning the opening 1202 as a slot antenna to emit a radiation noise through the opening 1202 at a specific frequency will be described with reference to FIGS. 11A and 11B. FIG. 11A is a cross-sectional view of the shielding casing 1202 taken in the line 11A-11A of FIG. 10. FIG. 11B is a diagram schematically showing a state in which currents flow in an external surface of the shielding casing 1202 around the opening 1202.
As shown in FIG. 11A, radiation noise 1211 is emitted from a noise source 1210 such as a printed circuit board. Radiation noise 1211 enters a ground current C2 flowing in an internal surface of the shielding casing 1201, and the ground current C2 becomes a current having a noise component. The ground current C2 having the noise component flows into the external surface of the shielding casing 1201 by taking a tuning path through the shielding casing opening 1202. As shown in FIG. 11B, the ground current C2 flowing into the external surface of the shielding casing 1201 flows through paths R11→R12→R13 in the external surface of the shielding casing 1201 detouring the opening 1202.
At this time, the ground current C2 causes standing waves at such a frequency that the ½ wavelength is equal to the length (D1) of the longer sides of the shielding casing 1201. Along the upper side of the shielding casing opening 1202, a standing wave having a “+” phase is generated. Along the lower side of the shielding casing opening 1202, a standing wave having a “−” phase is generated. That is, standing waves having a phase difference of π are generated close to each other between the upper and lower sides of the shielding casing opening 1202, thereby generating an electric field large in the vertical direction between the upper and lower sides. As a result, a signal having the frequency at which the ½ wavelength is equal to D1 resonates and become radiation noise, which is radiated out of the shielding casing 1201.
The noise component at a specific frequency when becomes a problem is a noise component having a frequency at which the ½ wavelength is equal to the length (D1) of each side of the opening 1201. Conventionally, the generation of radiation noise from the opening 1202 functioning as a slot antenna is prevented by setting the length (D1) of the longer sides of the opening 1202 to a value equal to or smaller than the ½ wavelength at the maximum of frequencies at which a regulation is required.
For example, when the length (D1) of the longer sides of the opening 1202 is 30 cm, radiation noise of 500 MHz at which the ½ wavelength is 30 cm is generated. Therefore, prevention of radiation noise of a frequency equal to or lower than 500 MHz requires setting the length of the longer sides of the opening 1202 to 30 cm or less.
Standing waves are generated along the shorter sides corresponding to the length (D2) of the shorter sides in the same manner as those along the longer sides to emit radiation noise. However, the frequency of radiation noise generated in association with the shorter sides is much higher than that of radiation noise generated in association with the longer sides. Therefore, not radiation noise generated in association with the shorter sides but radiation noise generated in association with the longer sides is a consideration.
Japanese Patent Application Laid-Open No. H08-32762 discloses a structure for shielding noise in an optical system. FIG. 12 is a perspective view of the structure. In FIG. 12, reference numeral 1301 denotes a shielding casing, and reference numeral 1302 denotes an opening in rectangular form for passage of light or paper having two shorter sides and two longer sides. The length of the longer sides of the opening 1302 is D1 and the length of the shorter sides is D2. A tube-shaped electroconductive member 1303 having substantially the same sectional shape as that of the opening 1302 and having a length D3 is attached inside the opening 1302. The tube-shaped electroconductive member 1303 is capable of preventing a radiation noise leaking from the interior of the shielding casing 1301 through the opening 1302 by utilizing a low-frequency-range shielding characteristic of a waveguide. That is, the radiation noise (electromagnetic waves) leaking out through the opening 1302 is reflected on an internal surface of the tube-shaped electroconductive member 1303, whereby the tube-shaped electroconductive member 1303 thus functions as a noise filter.
The principle of emission of noise in the shielding casing 1301 shown in FIG. 12 will be described with reference to FIGS. 13A and 13B. FIG. 13A is a sectional view of the shielding casing 1302 taken in the line 13A-13A of FIG. 12. FIG. 13B is a diagram schematically showing a state in which a current flows in a tube-shaped end portion 1303a of the tube-shaped electroconductive member 1303 far away from the opening 1302 of the shielding casing 1301 shown in FIG. 12.
As shown in FIG. 13A, radiation noise 1311 is emitted from a noise source 1310 such as a printed circuit board Radiation noise 1311 enters a ground current C3 flowing in an internal surface of the shielding casing 1301, and the ground current C3 becomes a current having a noise component. The ground current C3 having the noise component flows into an outer surface of the tube-shaped electroconductive member 1303 and reaches the tube-shaped end portion 1303a. 
As shown in FIG. 13B, the ground current C3 flows through paths R21→R22→R23 in the tube-shaped end portion 1303a. At this time, a noise component of a frequency at which the ½ wavelength is equal to the length (D1) of the longer sides of the shielding casing 1201 in noise components of the ground current C3 causes standing waves corresponding to the shape of the tube-shaped end portion 1303a. Along the upper side of the tube-shaped end portion 1303a, a standing wave having a “+” phase is generated. Along the lower side of the tube-shaped end portion 1303a, a standing wave having a “−” phase is generated. That is, standing waves having a phase difference of π are generated close to each other between the upper and lower sides of the tube-shaped end portion 1303a, thereby generating a large electric field in the vertical direction between the upper and lower sides. A signal having the frequency at which the ½ wavelength is equal to D1 resonates and become radiation noise. This radiation noise passes through the tube-shaped end portion 1303 and the opening 1302 to be radiated out of the shielding casing 1201, because the tube-shaped electroconductive member 1303 has substantially no effect of preventing passage of electromagnetic waves having frequencies equal to or higher than 500 MHz.
A part of the ground current C3 reaching a tube-shaped end portion 1304 flows into the internal surface of the tube-shaped electroconductive member 1303 and into the external surface of the shielding casing 1201 by taking a roundabout path. Ground current C3′ flowing into the external surface of the shielding casing 1201 downstream of the roundabout path flows through paths R11→R12→R13 in the external surface of the shielding casing 1301 detouring the opening 1302, as does the current shown in FIG. 11B. As a result, a signal having the frequency at which the ½ wavelength is equal to D1 resonates and become radiation noise, which is radiated out of the shielding casing 1301.
However, the upper limit of frequencies under noise regulations has been increased year after year. For example, when the necessary maximum shielding frequency is 2 GHz, the corresponding wavelength is 15 cm. It is necessary to set the length (D1) of the longer sides of the opening to a value equal to or smaller than ½ of the wavelength corresponding to the necessary maximum noise shielding frequency, i.e., 7.5 cm or less in the case of the shielding casing shown in FIG. 10. It is not practical to apply this condition to the opening through which light or paper is passed in the above-described office machine, because the opening is made too small to use.
In the shielding casing disclosed in Japanese Patent Application Laid-Open No. H08-32762, the tube-shaped electroconductive member 1303 is attached to the opening from the inside of the shielding casing and the low-frequency-range shielding characteristic of a waveguide is utilized to prevent leakage of radiation noise through the opening 1302. The low-frequency shielding characteristic is effective in preventing radiation noise of comparatively longer wavelengths, i.e., frequencies of 500 MHz or less, and the preventing effect is considerably reduced if the frequency is increased. Therefore, substantially no preventing effect can be obtained with respect to radiation noise of 1 GHz or 2 GHz radiated from the slot antenna.